It is well known in the field of optics that drive motors, such as stepper and servo motors, are coupled to gear-trains or belts, which drive iris diaphragms.
Stepper motors are electromagnetic devices designed to convert a series of input power pulses into discrete angular movements. Stepper motors are, however, also capable of continuous rotation. In contrast, servomotors are alternating current (AC) induction motors with electrically wound magnetic cores and low-inertia high-resistance rotors. Servomotors are designed for use in feedback control systems and for operation with applied continuous voltages.
Typically, an iris diaphragm is mounted in a camera or a telescope to restrict entry of light. Examples of cameras are still cameras, movie cameras, camcorders and the like. When taking a picture of an object, for example, it is necessary to control the opening and closing of the iris diaphragm in accordance with the magnitude of the luminance signal received therein. In other words, the larger the magnitude of the luminance signal, the smaller the aperture of the iris diaphragm required. Alternately, the smaller the magnitude of the luminance signal, the larger the aperture of the iris diaphragm required.
The drive motor drives the iris diaphragm to vary the size of the iris aperture. The drive motor operates in response to signals received from a motor control circuit. In order to generate signals to adjust the iris, the controller compares, for example, light from a scene being viewed by a telescope to a predetermined standard. When a difference exists between the light passing through the iris and the predetermined standard, a control signal is provided to the motor control circuit to generate a signal to drive the iris drive motor.
Gears are well understood components of many drive systems that tend to have inherent drawbacks. For example, friction, caused by loads, reduces efficiency. Contamination of other drive system components occurs from lubricant, which is used to reduce friction. Nonlubricated non-metal gears also contaminate drive systems through particulates, which are created from component wear.
An example of a current drive device for an iris diaphragm in an optical system includes a drive motor with a spur gear having a relative gear ratio to the gear teeth on the iris diaphragm. The rotor portion of the drive motor, connected either directly to the spur gear or to a spiral rail on which the spur gear moves, rotates in response to the iris control system.
The aforementioned examples require gear reductions between the rotor and the diaphragm. Fewer gear reductions would substantially decrease part count and potential drive system complexity.
An alternate example of a current automatic control device for controlling an iris diaphragm is a system through which the motor drives a belt, which drives the diaphragm. This design is, however, subject to belt wear, which can interfere with motor to diaphragm torque transfer efficiency.
Another known, adjustable iris system includes a manually adjustable iris, wherein the operator manually adjusts the opening of the iris in order to set the intensity of light received. Manual adjustment is, however, time consuming and difficult to calibrate.
In addition to the aforementioned, proper sizing of the drive motor is critical. The drive motor must be of sufficient size to avoid stall conditions and sufficiently scaled to avoid large vibration interference in the motor and to fit into product space requirements. A first major difficulty encountered through motor undersizing is that the torque on the iris, as it closes, approaches a value (i.e. infinity) at which the blades deform or tear. A second major difficulty encountered through motor undersizing is that the moment of inertia of the iris ring is substantial. A third major difficulty encountered through motor undersizing is that the gear-train requires a high gear ratio to drive the iris.
The disadvantages associated with current iris drive techniques have made it apparent that a new technique to drive iris diaphragms is needed. The new technique should substantially reduce part count, particularly the required number of gears, and should minimize the space occupied by the drive motor and the iris without impairing functionality. The present invention is directed to these ends.